According to a new report based on declassified Cold War-era spy satellite film, Himalayan glaciers have been receding twice as fast in the past 16 years as they did at the end of the 20th century.
The analysis, spanning 40 years of satellite observations across India, China, Nepal and Bhutan, indicates that glaciers have been losing the equivalent of more than a vertical foot and half of ice each year since 2000 — double the amount of melting that took place from 1975 to 2000.
"This is the clearest picture yet of how fast Himalayan glaciers are melting over this time interval, and why," said lead author Joshua Maurer, a Ph.D. candidate at Columbia University's Lamont-Doherty Earth Observatory.
The amount of ice loss from 2000 onward "is enough to fill 3.2 million Olympic size swimming pools," said Maurer.
The results, which are consistent with increasing temperatures in high-mountain Asia, are published in the June 19 issue of Science Advances. They offer a convincing indication that climate change is impacting even glaciers in the highest mountains of the world
"Accurate projections are still an area of ongoing research, but overall, the latest studies indicate that a substantial portion of glaciers in the Himalayas will be gone by the end of this century," said Maurer.
Currently harboring some 600 billion tons of ice, the Himalayas are sometimes called the earth's "Third Pole." Shrinking Himalayan glaciers pose major challenges to local societies, threatening water supplies for hundreds of millions of people downstream across much of Asia and increasing the risk of "meltwater lakes" building up rapidly and dangerously behind natural dams of rocky debris before spilling forth.
"Glacial lake outbursts floods are currently one of the greatest risks in this region," said Maurer. He highlighted a large flood in 1994 in Bhutan that resulted in fatalities and significant destruction of infrastructure.
Many recent studies have suggested that the Himalayan glaciers are wasting, but observations have been somewhat fragmented, focusing on shorter time periods, individual glaciers or certain regions. These studies have also produced contradictory results regarding the degree of ice loss and the causes. In particular, questions have persisted about these structures' sensitivity to climate change versus other drivers of ice loss.
"It is difficult to accurately pin down drivers of glacier change," said Maurer, "because the region is so complex — climatically, topographically and politically. It is difficult to access and measure these glaciers. And the extreme topography also presents unique challenges to numerical models."
Maurer and his colleagues took advantage of recently declassified, Cold War-era spy satellite film — which provides the earliest known images of the region — and modern NASA satellite imagery to quantify changes in ice thickness in Himalayan glaciers in the past 40 years. "Glaciers can gain or lose mass differently each year depending on annual climate fluctuations," said Maurer. "Thus, a long-term average is a more robust measure of the overall trend in ice loss."
Unlike past studies, their study used data from across the region, spanning 2000 kilometers (1242.7 miles) from west to east, in India, China, Nepal and Bhutan.
The researchers created an automated system to turn 20th-century observations from U.S. spy satellites into 3D models that could show the changing elevations of glaciers over time. They then compared these images with post-2000 optical data from more sophisticated satellites, which more directly measure elevation changes.
By analyzing repeat satellite images of some 650 Himalayan glaciers representing 55% of the region's total ice volume, the researchers estimate that, of the region's total ice mass present in 1975, 87% remained in 2000, and 72% remained in 2016. Glaciers across the region lost an average of about 0.25 meters (10 inches) of ice each year from 1975 to 2000, they say, and about half a meter (20 inches) annually starting in 2000.
Compared to ice loss in other major glacier regions like the Alps, say the authors, this ice loss is less severe, although it still shows accelerated ice loss over the past 40 years.
"The synthesis indicates that the melting is consistent in time and space, and that rising temperatures are to blame," said Maurer, noting that he and his team analyzed temperature data compiled from meteorological stations in high-mountain Asia during the study period. Their calculations of ice melt expected under these observed temperature increases match ice loss that actually happened. "It looks just like what we would expect if warming were the dominant driver of ice loss," Maurer said.
Average temperatures in the region (measured annually) increased about 1°Celsius (1.8° Fahrenheit) between 2000 and 2016, the data show.
While climate warming is likely the primary driver of accelerating Himalayan ice loss, other previously proposed effects may also play a role, Maurer and colleagues say. These include changes in precipitation, which seems to be declining in some areas and would tend to reduce the ice. Increased burning of fossil fuels by Asian nations, which sends soot into the sky and onto snowy glacier surfaces where it absorbs energy and hastens melting, may also have an influence, Maurer said, though this effect is highly variable throughout the region.
Some 800 million people depend in part on seasonal runoff from Himalayan glaciers for irrigation, hydropower and drinking water. The accelerated melting appears so far to be swelling runoff during warm seasons, but scientists project that this will taper off within decades as the glaciers lose mass. This will eventually lead to massive water shortages, they say.
"More arid regions in the Indus watershed will likely be hit the hardest," said Maurer.